GENEVA — Carmakers showed off their leanest, greenest mass-market models Thursday in anticipation of stricter European emissions controls as the Geneva International Motor Show opened its doors to the public.

A host of small cars are celebrating their debut in Geneva, including production versions of Ford Motor Co.’s new, fuel-stingy Fiesta and Toyota Motor Corp.’s three-seater iQ, and a concept version of Fiat’s 500 Aria. All three boast carbon dioxide emissions of under 100 grams per kilometer, which will help those manufacturers beat an EU proposal for a fleet average of 130 grams by 2012.

Environment ministers from the 27-nation bloc — the world’s biggest consumer market with 490 million people — met in Brussels earlier this week to discuss the plan, which has ruffled feathers among Europe’s large car-producing nations such as Germany.

The environmental group Greenpeace says even tighter emissions targets than those in the EU proposal are needed if climate change is to be reigned in.

About 20 activists protested at the show on Thursday morning, demanding that automakers rethink what makes a good car and calling for average fleet emissions to be limited to 120 grams per kilometer by 2012, and 80 grams by 2020.

The Geneva show is traditionally regarded as the most level playing field for car makers because Switzerland lacks its own industry.

Europeans will get their first glimpse of Indian manufacturer Tata’s Nano, hailed as the world’s cheapest car and due to begin rolling off the production line by the end of the year. The pint-sized five-seater will cost $2,500 plus tax and delivery, but will initially only be available in India while Tata works on improving the car’s emissions ratio and safety features.

Chevrolet too will be appealing to Europeans accustomed to driving smaller vehicles than their trans-Atlantic cousins.

General Motor Corp.’s top-selling brand is unveiling a three-door version of the Aveo, which debuted in Europe at last year's IAA exhibition in Frankfurt, Germany, with a five-door model.

One of the loopholes to the emissions proposal currently being discussed in Brussels lets car makers pool their fleets with those of companies producing more efficient vehicles.

Companies that exceed the limit would face stiff fines starting at $30 (20 euros) per gram per kilometer, multiplied by the number of cars sold, rising to $144 (95 euros) by 2015. If auto makers pass this cost on to consumers it would add hundreds of euros to the price of a car.

Low-volume manufacturers producing fewer than 10,000 vehicles a year would be exempt, but major players such as BMW, Daimler and Volkswagen would have to team up with greener car makers or improve their own fleet if they want to continue selling high-powered SUVs and sports cars in Europe.

The EU proposal, even if it is watered down before being agreed upon, has set automakers fine-tuning their product range to reduce emission levels as much as possible.

Moves by several large cities to follow London’s lead in charging owners of gas-guzzling vehicles a daily fee to enter the city have also spooked manufacturers and given them a further incentive to reduce emissions.

London mayor Ken Livingstone last year raised the daily charge to $49 (25 pounds) for cars that produce more than 225 grams of CO2 per kilometer, while those that produce less than 120 grams per kilometer enter free of charge.

Matthias Wissmann, head of the German auto industry group VDA, has accused Livingstone of appealing to “social envy” rather than pursuing a sound environmental goal.

Almost every major manufacturer in Geneva is showing off hybrid versions that use electricity or hydrogen in addition to conventional fuels. Lighter materials, electricity-generating brakes and even solar panels will be on show by the 260 exhibitors.

No single technology has yet emerged as the definitive way of reducing emissions, but manufacturers including General Motors and Daimler have put their weight behind developing more efficient lithium-ion batteries for use in hybrid vehicles. The German car maker said its Mercedes-Benz flagship S-class luxury sedan will be available in a gas-electricity hybrid version next year.

Martin Lloyd of Greenpeace, who toured the show before its official opening, accused car makers of “greenwashing” their industry with a few headline-grabbing eco models while continuing to sell heavier, faster and less efficient vehicles.

“The industry knows what it has to do and it has the technology to do it,” Lloyd said.